Quick Hits

📱 EPISODE 3 — Article 245 (Part 3)

Power Fuses & Fuseholders

SLIDE 1 — Fuse Use Basics (245.21(B)(1))
🔥 POWER FUSE FUNDAMENTALS

When fuses protect conductors/equipment:
✅ A fuse goes in EACH ungrounded conductor
✅ Two power fuses CAN be used in parallel for the same load — but only if both have identical ratings AND are in an identified common mounting that divides current equally
🚫 Vented-type power fuses can't be used indoors, underground, or in metal enclosures — unless specifically identified for that use

SLIDE 2 — Ratings & Identification (245.21(B)(2)–(4))
📊 FUSE RATING RULES

🔹 Interrupting rating ≥ available fault current the fuse must interrupt (all sources included)
🔹 Max voltage rating ≥ max circuit voltage — and if a fuse has a MINIMUM recommended operating voltage, don't apply it below that either
🔹 Fuse mountings & fuse units need permanent, legible markings: manufacturer type/designation, current rating, interrupting rating, voltage rating

Also — fuses that expel flame when opening must be designed/arranged so they operate safely without hazarding people or property (245.21(B)(5)).

SLIDE 3 — Fuseholder Safety & Medium/High-Voltage Fuses
⚠️ FUSEHOLDER SIGN REQUIREMENT (245.21(B)(6))

Fuseholders must be de-energized during fuse replacement — UNLESS designed for qualified persons to replace fuses using identified equipment without de-energizing.

A permanent sign is required next to fuseholders:
"DANGER — DISCONNECT CIRCUIT BEFORE REPLACING FUSES."

🔌 MEDIUM/HIGH-VOLTAGE FUSES (245.21(B)(7))
Switchgear/substations using these fuses need gang-operated disconnecting switches — load-interrupter type, unless interlocked with a load-interrupting device. Isolation happens via a switch between source and fuses, OR roll-out switch/fuse construction.

SLIDE 4 — Exam Question + CTA
🧠 TEST YOURSELF

Q: Per 245.21(B)(1), where fuses protect conductors and equipment, how many fuses are required per ungrounded conductor (minimum)?

A) Two fuses per conductor
B) One fuse in each ungrounded conductor
C) One fuse total for the whole circuit
D) Fuses are optional if breakers are also used

(Drop your answer 👇)

Next up: Distribution cutouts and oil-filled cutouts. FOLLOW for more ⚡ #JoshTheSparky #NEC2026 #ElectricianLife

← View Chapter 2 Home

← Back To Article 245 Home

← Back To Quick Hits Home